Identification of sound and like signals



Oct. 10, 1961 E. F. HEMBROOKE IDENTIFICATION OF SOUND AND LIKE SIGNALSFiled April 29, 1954 mnPC ESZ FREQUENCY f' looo IOO . E M T moat E24LOUDSPEAKERW AMPLIFIER :PLAYBAOK' RECORD l l. l

' FILTER \NVENTOR Emil Fran/r, Hemraoie BY MWMf I! 7 ATTORNE qTRANSDUCER EILTER 3,004,104 IDENTIFICATION OF SOUND AND LIKE SIGNALSEmil Frank Hembrooke, Brooklyn, N.Y., assiguor to Muzak Corporation, NewYork,'N.Y., a corporation of New York Filed Apr. 29, 1954, Ser.'No.426,465 6 Claims. (Cl. 1792) 'vides for the identification of recordedmusic or other audio signals by coded signals which are not evident to alistener but which nonetheless can easily be detected and which are suchan integral part of the audio signals.

that they are difficult if not impossible to obliterate.

The unauthorized recording and rebroadcasting of musical performances,for example, has been difiicult to prevent because of the inability todetect positively whether the rebroadcast is an exact reproduction ofthe original or a different performance. tion makes possible thepositive identification of the origin of a musical presentation andtherebyconstitutes an effective means of preventing such piracy, ie itmay be likened to a watermark in paper.

The invention may be used for the identification of any kind of signal,whether audio, or other, comprising a number of dilferent frequencycomponents, although for purposes of illustration it is shown anddescribed herein as applied to the identification of audio signals.

It is well known that a complex audio signal, such as music or speech,is composed of a number of frequency components extending over a widerange of frequencies.

In accordance with the present invention such a signal is identified bysuppressing a selected frequency, or narrow band of frequencies, withinthe frequency spectrum of the signal at timed intervals accordingtoapredeterimined code. Because of the insensitivity of the ear indetecting the absence of a particular frequency or narrow band offrequencies in a sound signal, the identifying code will beimperceptible to the listener.

The suppression of a selected frequency or narrow band of frequencies isaccomplished, in a specific embodiment of the invention, by convertingthe signal into a complex electrical voltage and then passing thisvoltage through a very narrow band-reject electrical filter which passesall but a very narrow range of frequencies in the complex voltage.

In the accompanying drawings:

FIGURE 1 is a graph of the spectrum of frequencies in a typical soundsignal at an instant of time when a predetermined narrow range offrequencies is being suppressed;

FIGURE 2 is a graph of the amplitude of the sound signals in saidpredetermined narrow range of frequencies, plotted versus time toportray an illustrative coding sequence;

FIGURE 3 is an electric circuit, in block form, for impressing anidentifying code on a signal in accordance with one embodiment of thisinvention;

FIGURE 4 is an electric circuit, in block form, for detecting theidentifying code.

In FIGURE 1 the horizontal scale represents the fre- The presentinvenquencies of the various components in a typical sound signal whilethe vertical scale represenst the amplitude of these various componentsat a particular instantof time. The audible frequency range of the soundsignal, as indicated in this figure, extends approximately from 20cycles per second to 20,000 cycles per second.

The amplitudes of frequency components of the signal over a very narrowrange of frequency, for example, 10 cycles per second, at some place inthe frequency spectrum are attenuated, in accordance with the invention,

in order to identify the signal. The attenuation of these.

for example, in the vicinity of 1000 cycles per second,

that can be accommodated in even the cheapest reproducing systems sincethe permanence as an identifying mark of the attenuated harmoniccomponents when so placed is greater than if frequency i were placed atone edge of the signal frequency range. If it were placed at one edge,for example, at approximately 20,000 cycles per second, the identifyingeffect could be entirely eliminated simply by filtering out all of thevery high frequencies of the sound signal. Moreover, in this regard, thefrequency i should be placed at a frequency which is generally presentin the signal to be identified. ,This frequency will of course dependupon the signal, but for music approximately 1000 cycles per second hasbeen found to be satisfactory.

As shown in FIGURE 2, the identifying mark is discontinuous andeliminates the selected frequency components of the signal only atcertain intervals of time, according to a predetermined, coded pattern.

FIGURE 3 shows a specific illustrative embodiment of a coding apparatusaccording to the invention comprising an electric circuit for impressingupon the signal to be marked a code of the type illustrated in FIG- URE2. The audio signal voltage, comprising a number of frequency componentsextending over a wide range of frequency, is applied at input terminal11. This voltage is switched by means of an encoding switch 12' eitherdirectly from terminal 11 to terminal 13 or through a band-reject filter14 to terminal 13, depending upon whether the switch,12 is open orclosed. The switch 12 is opened and closed in sequence according to apredetermined coding pattern, for example, the name of the orchestra orthe trademark of the recording company may be spelled out inInternational Code, or any other suitably identifiable sequence ofpulses may be utilized as a proprietary code. The encoding switch may beactuated mechanically, as by means of a coding cam, so that the codingequipment is fully automatic and may be employed continuously at veryslight expense. The band-reject filter 14 preferably has a very high Qthat is, it is sharply tuned to reject only the frequency componentswithin an extremely narrow range on either side of its resonantfrequency, 13,, so that the absence of the rejected frequency componentswill be imperceptible to the listener. This necessitates, of course,that the reduction in the total power of the audio signal due to thedeletion of said selected frequency components be less thanapproximately 3 decibels.

The output voltage from terminal 13 may be broadcast concurrently or itmay be recorded and reproduced at. a later time, as illustrated inFIGURE 3, by means of.

a playback head, amplifier and speaker.

The code impressed on the signal by switch 12 will serve to identifythereafter the origin of the audio signal so marked.

FIGURE 4 shows a specific illustrative embodiment Patented Oct. 10,1961v of an apparatus for detecting the identifying code in the audiosignal. This apparatus includes an input terminal 20 to which may beapplied an audio signal voltage similar to that obtained at terminal 13or terminal 13 in FIGURE 3. This voltage is amplified by means of anamplifier 21 and fed through a band pass filter 22, which has atransmission characteristic such that it will pass. only frequencycomponents within the same narrow range of frequency which theband-reject filter 14 will reject, as previously described. In'otherwords, the transmission characteristics of the band-pass filter 22' aresubstantially opposite to those of the bandreject' filter '14. Theoutput of the band-pass filter 22 is connected to a suitable transducer23, such as a recording meter, oscilloscope, Or the like. As the voltageapplied to terminal 20 passes through the filter 22, all frequencycomponents of the signal lying outside the pass band are eliminated andonly those within this band are passed. Since these frequencies werealternately suppressed and passed by the encoding circuit illustrated inFIGURE 3, the voltage applied to the transducer 23 will be intermittent.If transducer 23 is a recording meter containing a rectifier whichconverts these audio frequency alternating current voltages to directcurrent voltages, it can plot a graph oftheir amplitude versus time.Thus, a graph of the type illustrated in FIGURE 2 is plotted, in whichthe coding pattern, as indicated at 24 in FIG- URE 2, is readilyapparent. Alternatively, transducer 23 may be a loudspeaker to convertthe identifying code into sound.

The foregoing is intended in illustration and not in limitation. Changesor modifications inthe embodiments illustrated will occur to thoseskilled in the art and these changes or modifications may be madewithout departing from the spirit or scope of the invention as setfoith.

' I claim:

1. In combination, an input terminal, means for supplying to saidterminal an electric signal corresponding to an original signal andhaving a plurality of frequency components each having a respectivefrequency and amplitude, an output terminal, encoding means for impressing a distinctive code on said electric signal for identifying theorigin of said original signal, said encoding means including anarrow-band-reject filter adapted to attenuate said components of saidelectric signal within a very narrow frequency range, and also includinga switch for connecting and for disconnecting said filter between saidterminals in a predetermined time sequence to form said distinctivecode.

2. A system of reproducing a continuous sound such as a musicalselection and of permanently but unchtrusively identifying itsorigincomprising: means for continuously generating an electric signalcorresponding to said sound and having frequency components extendingsubstantially over the audio range of frequencies, means for variablyattenuating in accordance with an identifying code frequency componentsof said signal lying within a very narrow band of frequency within saidaudio range, and means for then utilizing said identifiedelectric-signal whereby either said sound substantially in original formwithout the audible presence of said code or said code alone can bereproduced.

3. The system as in claim 2 wherein said' means for utilizing includesmeans for eliminating all frequency components of said electric signalexcept those remaining in said narrow band. 1

4. The system as in'claim 2 wherein said narrow band of frequency has awidth of only a few cycles per second, and is near the center of saidband.

5. A method of unobtrusively identifying a sound signal such as amusical selection, said method'comprising the steps of taking a soundsignal having frequency components within the audio range, andattenuating the components in a very narrow frequency band within saidrange in accordance with an identifying pattern so that said signal willbe permanently marked with an easily detectable pattern but a personhearing said marked-- signal will be unaware of audible change in it.

6. The method as in claim 5 wherein said narrow frequency band has awidth of the order of ten cycles per second and lies in said audio rangenear a frequency of one thousand cycles per second.

References Cited in the file of this patent UNITED STATES PATENTS1,724,938 Jammer Aug. 20, 1929 2,116,172 Hyneman et al May 3,. 19382,335,335 Zenner Nov. 30, 1943 2,352,918 Smith July 4, 1944 2,376,275Rhoads May 14, 1945 2,398,755 Shepherd Apr. 16, 1946- 2,406,034 PhelpsAug. 20, 1946 2,474,191 Reid et al. June 21, 1949 2,503,701 BaughmanApr. 11, 1950 2,580,973 Sueur Jan. '1, 1952 2,636,936 Goldsmith Apr. 28,1953-

